Photovoltaic technology is poised to play an important role in the development of renewable energy sources. The largest volume application for photovoltaic (PV) cells is expected to be in power generation for commercial, industrial and domestic use. Although the past few years have seen increased deployment, fueled in part by skyrocketing oil prices, PV today accounts for less than 0.1% of power generation.

The major barrier to widespread adoption of PV cells is the cost of each cell compared to the energy it can generate, which is a function of material cost, manufacturing cost and cell efficiency. The good news is that – since this is not a mature technology – there is plenty of room for improvement of these factors for all the technologies currently used.

Those countries seeing the most significant growth in PV are the ones that have used preemptive initiatives to make solar power attractive to the consumer before its cost reaches parity with more conventional forms of power. Japan, for example, had a program dubbed “One Million Rooftops,” an initiative to have PV panels mounted on the roofs of one million Japanese homes. Once this goal was reached, however, the initiative was disbanded, and the Japanese PV market fell into a lull.

Today, Germany and Spain are hotbeds of activity. The programs in these countries are based on feed-in tariffs, where those that install PV systems will be permitted to return electricity to the grid to receive a guaranteed stipend. This sort of initiative has met with far greater acceptance because the payback is more solid than what was offered in Japan.

Most of the attention on today’s PV development is focused on the conversion technology – the semiconductor that actually produces the electricity. The efficiency of the final PV panel is a function of the technology used. This portion of the system accounts for about half the expense and is the part of the supply chain where innovation is most likely to occur. The other half (or more) of the cost goes to assembly of PV cells into modules (10 to 15%) and installation costs (35 to 40%), which are predominantly labor.

Two main semiconductor technologies are currently used for direct solar conversion to electricity:

Crystalline silicon – monocrystalline and polycrystalline.

Thin films – amorphous silicon (aSi or a-Si), cadmium telluride (CdTe) and copper indium gallium selenide (CIGS).

In addition, promising work is being done with organic materials and dye-sensitized cells, but these technologies are still in development. It is unclear at this time which technology (or technologies) will take leadership of the market in the future, and it is possible that we will see new technologies displace all of these technologies over the next 15 years. The market is too young to tell.

Cell Efficiency

Perhaps the most important issue to be addressed for PV cells is their efficiency. The higher the cell’s efficiency, the more cost-effective it becomes, since more power can be from a given area of active material. Significant research is underway to improve the efficiency of PV cells (Figure 1). Multijunction cells (the purple line) are clearly the most efficient. These devices use multiple P/N junctions (positively and negatively doped semiconductors) to generate electricity from different wavelengths of light (Figure 2). In these cells, secondary junctions scavenge energy that passes through the first junction, usually taking advantage of the spectral sensitivities of two complementary solar cell technologies. Production costs for multijunction cells are currently expensive, but it is reasonable to expect them to fall over time.

Below multijunction cells fall the more conventional silicon technologies: monocrystalline, showing the highest efficiency, followed by multicrystalline. The efficiency of monocrystalline silicon cells can be increased with through use of concentrators (reflectors or lenses) (shown by the dotted line).

Even lower in efficiency are the thin-film technologies (green lines), which promise to be far less costly to produce than any technology manufactured on an expensive purified silicon substrate. Should the lower cost offset the loss in efficiency, thin film PV cells would likely be used for cost-sensitive products, with silicon-based technologies reserved for niche markets where higher cost per watt is acceptable in light of space limitations.

The red lines denote research on organic and dye-sensitized cells. Since these technologies expect to harness volume-printing techniques, they are likely to be the lowest cost to produce, possibly resulting in a very low cost per watt. It will be a number of years before we can clearly see whether this technology can compete favorably against its inorganic competition.

Today, higher efficiency comes at a higher cost. For example, high-efficiency multijunction technology costs more than monocrystalline, which, in turn, is more expensive than polycrystalline. Even less expensive than polycrystalline are thin films, and organic cells are the least expensive (but also least efficient). Given these tradeoffs between efficiency and cost, cost per watt is the most rational way to compare the technologies. Ongoing development for all of these technologies is focusing on materials and processes that will reduce the costs and/or improve efficiencies.

Silicon. An estimated 90% of solar cells in production today are made of silicon. Silicon has the advantage of being better understood than other materials, and volumes are significantly higher. The process used for crystalline PV cells is a subset of that used for standard semiconductor processing, so manufacturing efficiencies are high.
Monocrystalline wafers are manufactured via the same crystal-pulling technique used to make wafers for standard semiconductor processes. This approach is somewhat more expensive than the molding approach used for polycrystalline wafers. However, polycrystalline wafers have lower peak efficiency; thus there is a cost/efficiency tradeoff between these two approaches.

Thin films. A recent silicon shortage drove development of thinner wafers and amorphous silicon cells. It also helped encourage the development of alternative thin film materials, which are attractive for two reasons:
They can be manufactured in unusual shapes (curves), on flexible substrates, and by continuous rather than batch processes (roll-to-roll).

They reduce the industry’s dependence on bulk silicon.

Thin film cells consist of active semiconductor material that has been deposited on a passive substrate – most commonly, glass, stainless steel or plastic. This differs from the approach used with crystalline silicon cells in which an active region is formed into a substrate of bulk semiconductor material. The amount of active material can be significantly reduced if it is not used as a substrate to provide mechanical support.

The leading thin-film PV technologies include amorphous silicon (aSi or a-Si), cadmium telluride (CdTe) and copper indium gallium diselenide (CIGS). To date, CdTe and CIGS exhibit the highest efficiencies of any single-junction thin film materials.

Organic PV devices. Countless alternative PV technologies are being researched. One that holds promise as a favorable alternative to existing technologies is the organic PV cell, a new thin film technology. This is a cell whose active element is an organic material, as opposed to the inorganic materials used in silicon, CIGS, and CdTe cells.

Organic cells are attractive for three reasons:

• They can be manufactured by low-cost screen-printing processes or even inkjet printers.
• They can be printed onto flexible substrates, permitting use of very inexpensive materials and simplified handling.
• They can be used to make a lightweight power source for portable products.

Organic photovoltaics had a slow start since early experiments showed efficiencies below 0.1%. The use of nanostructured material cells led to more efficient charge separation, and efficiencies are currently in the 3% to 5% range. Work in this area is still primarily research-based at universities and institutes and a few pioneering startup companies.

Applications. Applications range from grid-connected to completely distributed electricity generation. Grid-connected could be residential (on rooftops of homes) to commercial (rooftops of retail stores and office buildings) to utility scale (public utilities using PV).

Different technologies have better suitability to different latitudes and climates. For example, concentrated solar is best in direct sunlight, close to the equator, using tracking technology. Thin films (silicon or others) are better suited for northern latitudes and somewhat cloudy climates. Single crystal silicon does well in mid latitudes.
A new class of applications is emerging – building-integrated photovoltaics (BIPV) – for which flexible substrates are well suited. With BIPV, special photovoltaic materials replace conventional building materials in parts of a building – such as the roof, skylights or facade – as an alternative to traditional PV modules mounted above the roof on racks (Figures 3 and 4). These applications are expected to grow significantly in the next several years. NanoMarkets projects the market for BIPV will exceed $4 billion in revenues by 2013 and surpass $8 billion in 2015.¹

Long-Term Challenges

Photovoltaics have been a low-volume technology over their history, but we anticipate significant changes over thenext 15 years, as the cumulative volume of solar modules increases significantly. This somewhat clouds the picture, making it difficult to predict what kinds of challenges we may face in the next decade and beyond.
We expect for some of the more mundane issues to be worked out – issues like materials shortages and the quantity of material required to manufacture a PV cell. These issues are typically related to commodity-like behavior and to supply/demand behaviors. Several other portions of the system will be fine-tuned to provide more efficient operation; for example, inverters and other control electronics will be incrementally improved.

It is not clear whether today’s standard silicon, CdTe and CIGS cells will yield to other technologies, or which of these three existing technologies might take the lead. Efficiency will not be the key measure of acceptance. Cost per watt will determine which technology wins and which others lose. As manufacturing volume increases, we will be able to discern which of these technologies proves to have a clear cost/watt advantage.

Unlike semiconductors, there appear to be no technical barriers to the acceptance of new PV technologies. Photovoltaics will be governed more by economics than by any performance specifications, and the economics of these devices depend on little more than manufacturing cost and efficiency.

References

NanoMarkets, “Building Integrated Photovoltaics Markets: 2008,” July 2008.

Alain Harrus is a partner with Crosslink Capital (crosslinkcapital.com) and co-chair of the Photovoltaics chapter of the 2009 iNEMI Roadmap; aharrus@crosslinkcapital.com. Jim Handy is a director at Objective Analysis (objective-analysis.com) and co-chair of the Photovoltaics chapter.

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